Conventional telephone circuits provide a frequency bandwidth of up to 3,300 Hz, which is quite adequate for normal voice conversations. Sometimes, the need arises for higher quality transmissions as, for example, in connection with radio broadcasts of conversations carried via telephone lines. When such a need arises and time permits, a specially conditioned line is provisioned by the telephone company to provide a wider bandwidth, e.g., from 300 to 5000 Hz. Conditioning of a telephone line is expensive and time consuming. This is particularly true when a telephone connection is provisioned through a variety of transmission equipment. A telephone connection can be implemented, for example, with a line pair to the central office, an analog carrier from one central office to another, and a digital carrier to still another central office.
In other applications where wide band service is desired, the broadband signal is separated into a plurality of sub-bands by the use of bandpass filters, each sub-band is shifted to baseband, and the baseband signals are transmitted to the destination over separate channels. On the receiving end, the signal of each channel is modulated up to its original frequency band position and the resulting plurality of band limited signals are arithmetically combined to develop the desired broadband signal. Such an approach can be easily applied to the task of obtaining a desired wider bandwidth in the aforedescribed application of radio broadcast transmissions over telephone lines. The problem with this approach, however, is that when transmission over one of the telephone lines is disrupted, an entire band of the original signal is missing at the receiving end, and it cannot be recovered; and when the missing band contains the low frequencies, intelligibility is greatly affected.
It is an object of this invention to provide for high fidelity audio transmission over relatively narrow band telephone lines without the loss of an entire frequency band when one of the telephone line connections fails.